Acrylic acid is widely used on an industrial scale as a raw material of water-absorbent resins and the like, and is commonly produced by two-step oxidation in a fixed bed multitubular continuous reactor. In the two-step oxidation, propylene is converted to acrolein by catalytic gas-phase oxidation in the presence of an oxide catalyst, and the resulting acrolein is subjected to catalytic gas-phase oxidation to produce acrylic acid. Since propylene is a raw material derived from fossil resources, production of acrylic acid from renewable resources is desired.
Additionally, an acrylic ester is produced by esterification of acrylic acid to be widely used as a raw material of various resins such as adhesives and coating compositions.
An attempt has been made for economically producing acrylic acid on a commercial scale from biomass that is a renewable resource. Examples of a method for producing acrylic acid from biomass include a method dehydrating hydroxypropionic acid (also referred to as HP) to prepare acrylic acid. Examples of hydroxypropionic acid include such as lactic acid (also referred to as 2-hydroxypropionic acid or 2HP) that is a natural product and is easily available and 3-hydroxypropionic acid (also referred to as 3HP) prepared by fermenting sugars obtained from natural products or obtained by decomposition of cellulose.
Patent Literature 1 discloses a method in which an aqueous or other solution containing β-hydroxycarboxylic acid or its salt obtained by fermentation or the like is prepared, and the solution is heated in the presence or absence of a dehydration catalyst for dehydration, thereby producing unsaturated carboxylic acid or its salt.
Patent Literature 2 discloses a method in which an aqueous solution containing α- or β-hydroxycarboxylic acid is introduced to a site retaining an inactive ceramic and the like or an acidic solid catalyst, and heated to prepare α, β-unsaturated carboxylic acid. Though Patent Literature 2 further teaches that an aqueous solution containing a polymer, oligomer, lactide, lactone, or the like produced from α- or β-hydroxycarboxylic acid may be used, no specific examples are mentioned.
In the case where acrylic acid is produced by vapor phase dehydration in which a raw material composition containing hydroxypropionic acid is evaporated and brought into contact with a catalyst, an accretion may be formed in an evaporator or reactor eventually to block the evaporator or reactor, problematically resulting in a failure in stable production for a long time. In addition, the catalyst surface covered with the accretion lowers the catalytic activity, problematically leading to lowering in the conversion of hydroxypropionic acid. As a result, the produced acrylic acid may contain unconverted hydroxypropionic acid. In such a case, problematically, the purity of the acrylic acid may be lowered or the hydroxypropionic acid may react with the acrylic acid to lower the yield of the acrylic acid in purification step after the reaction.
A hydrophilic resin, in particular, a water-absorbent resin is demanded to have a reduced residual monomer content from the viewpoint of its performance and safety. Accordingly an acrylic acid raw material used in preparation of water-absorbent resin is strongly demanded to have fewer impurities such as hydroxypropionic acid, dimers, and oligomers which may cause residual monomers in the water-absorbent resin (Patent Literature 3, Patent Literature 4). In the above methods, however, the resulting acrylic acid contains a large amount of unreacted hydroxypropionic acid monomers and heavy byproducts, and therefore, complicated purification needs to be performed. Consequently, the above methods are technically insufficient for obtaining acrylic acid to be used as a raw material of a water-absorbent resin and have room for improvement.